Jest

Jest is used by Facebook to test all JavaScript code including React applications.

Installation

npm i jest --save-dev

in package.json under "scripts" section

"scripts":{
  "test":"jest"
}

Create a folder named "tests", create a file named sample.test.js and write a sample test like this

test('Our first test', () => {

});

run the sample test form the command prompt

npm test

Usage

a simple test file

const lib = require('../lib');

test('absolute - should return a positive number if input is positive', () => {
  const result = lib.absolute(1);
  expect(result).toBe(1);
});

Grouping

const lib = require('../lib');
describe('GroupName', ()=>{
  it('should return this if that is entered', () => {
    const result= lib.absolute(1);
    expect(result).toBe(1);
  });  

  it('should return other if something else is entered', () => {
    const result= lib.absolute(2);
    expect(result).toBe(2);
  });  
});

Testing Exceptions

const lib = require('../lib');
describe('GroupName', ()=>{
  it('should throw if username is falsy', () => {
    // null, undefined, NaN, '', 0, false
    expect(() => {lib.registerUser(null)}.toThrow();
  });  
});

Continually Running Tests

in package.json under "scripts" section we add watchAll flag

"scripts":{
  "test":"jest --watchAll"
}

Jest Mock Function

const mockFunction = jest.fn();
// mockFunction.mockReturnValue(1);
// mockFunction.mockResolvedValue(1);
mockFunction.mockRejectedValue(1);
const result = await mockFunction();

db.getCustomerSync = jest.fn().mockReturnValue({ email:'a' });

mail.send = jest.fn();
expect(mail.send).toHaveBeenCalled();

for the arguements

expect(mail.send.mock.calls[0][0]).toBe('a'); //first arguement
expect(mail.send.mock.calls[0][1]).toMatch(/order/); //second arguement